The invention concerns a polyolefin based thermoplastic elastomer which can be prepared without a separate vulcanization stage and which has polyacrylate as a dispersed phase and which has been achieved by polymerization of acrylate into the polyolefin matrix.
Thermoplastic elastomers are polymers which have the desirable processing properties of thermoplastics but have the same physical properties a vulcanized rubbers. This combination of properties generates materials having segments that are soft and elastic with low glass transition temperature (tK) and a rigid, eventually crystalline, segment with a high glass transition temperature or a high melting point. The rigid and soft segments must be thermodynamically incompatible so that they form separate phases. Unlike conventional rubber, thermoplastic elastomers do not need a separate vulcanizing stage and can be processed using methods normally used with thermoplastics, such as extrusion, injection molding and blow molding. In addition, thermoplastic elastomers can also be reprocessed, for example when recycling material from the processing stage.
Thermoplastic elastomers can be divided into two main groups, block copolymers and thermoplastic/elastomer blends. A well-known example of block copolymers, which are thermoplastic elastomers, is the anionically polymerized block copolymer of styrene and butadiene (SBS) and the hydrogenized form of the same (SEBS). When these polymers are at room temperature, the soft and elastic phase is the continuous phase and the rigid phase, polypropylene, is dispersed. Here, the rigid polystyrene gives the material its strength, but during processing the temperature is raised over the glass transition temperature of polystyrene when it melts and the material can flow. The SBS thermoplastic elastomer, however, has poor weather resistance because of the butadiene double bonds. In SBS and SEBS polybutadiene and its hydrogenated form is the continuous phase, consequently they both have low oil resistance. Additionally, SEBS is expensive and requires a complicated preparation method.
Examples of materials that belong to the group of thermoplastic/elastomer blends are blends of polypropylene and ethylene/polypropylene rubber or ethylene/polypropylene/diene rubber. In these blends the rigid polypropylene phase is the continuous phase and the soft phase is dispersed, giving the material good oil resistance properties. These blends are made by blending the two main components and various additives in an extruder. Stabile phase separation results from curing the dispersed rubber phase (see, for example, U.S. Pat. No. 4,594,390).
The current invention describes a method to produce a thermoplastic elastomer with a polyolefin as a continuous phase and a rubber-like polyacrylate as a dispersed phase. This product is made in a reactor where crosslinking, if needed, can occur during polymerization. Thus, no separate vulcanization stage is needed. The resulting product has very good weather and oil resistance properties because the polyolefin is the continuous phase and because the elastomer is a polyacrylate. Hence, the current invention provides a method to produce a polyolefin based thermoplastic elastomer with a dispersed polyacrylate phase and without requiring a separate vulcanization stage.
The object of the current invention is to provide a new thermoplastic elastomer comprising a polyolefin/polyacrylate blend that has the polyolefin as the continuous phase and the polyacrylate as a dispersed phase.
A further object of the invention is to provide a new thermoplastic elastomer which maintains its dispersed polyacrylate structure during processing due to crosslinking of the dispersed elastic polyacrylate phase to the continuous polyolefin phase during polymerization.
Another object of the current invention is to provide a method for preparation of the new thermoplastic elastomer without employing a separate vulcanization stage.
The current invention provides for a polyolefin based thermoplastic elastomer with a dispersed polyacrylate phase that is polymerized into the polyolefin matrix. The invention further provides for a method for its preparation without a separate vulcanization stage. The acrylate used in the current invention has elastic properties and a glass transition temperature that is below room temperature. The acrylate forms a dispersed phase in the polyolefin matrix and, because polymerization occurs by the free radical technique, part of the acrylate chains are crosslinked to adjacent polyolefin chains. This provides good adhesion between the continuous polyolefin phase and the dispersed polyacrylate phase. Crosslinking can be controlled using varying ratios of diacrylate and acrylate. This crosslinking is especially important in cases where low adhesion between the polyolefin matrix and the polyacrylate is expected, for example when homopolyethylene or polypropylene is used. Here, because of the crosslinking, the soft dispersed polyacrylate is maintained in its dispersed form during processing when the polyolefin melts and becomes fluid.
The material could be produced by some of the methods given in the patent literature in which monomers are polymerized by free radical polymerization techniques into polyolefin matrix, e.g. by the Finnish patent 88170. In principal the acrylate monomer, and optionally diacrylate monomer, and the initiator are absorbed into polyolefin particles. The impregnation temperature is low enough so that no decomposition of the initiator occurs, yet high enough so that the monomer and the initiator can penetrate into the polyolefin particles. When all of the monomer and initiator have been absorbed, the temperature is elevated and the initiator decomposes and initiates the polymerization of the acrylate. The polyolefin particles swell to some extent (depending on the amount of monomer added) during the impregnation, but maintain their particle structure. The polyolefin particle structure is also maintained during polymerization.
Useful polyolefins include high density polyethylene, low density polyethylene and linear low density polyethylene. The polyethylene can be a homopolymer or a copolymer. The co-monomer of ethylene can be vinyl acetate, vinyl chloride, propylene or some other xcex1-olefin, C1-C7-alkylacrylate and -methacrylate, glycidylacrylate and -methacrylate, dienes such as hexadiene-1,4, hexadiene-1,5, heptadiene-1,6, 2-methylpentadiene-1,4, octadiene-1,7, 6-methylheptadiene-1,5 and polyenes such as octatriene and dicyclopentadiene. Also ethylene-xcex1-olefin-polyene-terpolymeres are useful. Useful xcex1-olefins include propylene, butene, pentene, isoprene, hexene or their mixtures and useful polyenes include hexadiene-1,4, hexadiene-1,5, heptadiene-1,6, 2-methylpentadiene-1,4, octadiene-1,7, 6-methyl-heptadiene-1,5, octatriene, dicyclopentadiene. In cases where an ethylene copolymer is used, at least 50% by weight must be ethylene.
The polyolefin can also be comprised of polypropylene and its copolymers. Propylene copolymers must consist of over 50% by weight propylene and can be random- or block copolymers of propylene and ethylene. Also, other xcex1-olefins can be used as co-monomers and also dienes such as hexadiene-1,4, hexadiene-1,5, heptadiene-1,6, 2-methylpentadiene-1,4, octadiene-1,7, 6-methylheptadiene-1,5 and polyenes such as octatriene and dicyclo-pentadiene.
The polyolefin can be in any form, but preferably in the form of pellets with a diameter of 0.5-10 mm. Particle forms of the polyolefin facilitate after treatment washing and drying.
Suitable monomers are acrylates and methacrylates whose polymers have low glass temperatures, that is, they are rubber-like at and below room temperature, preferably at temperatures below xe2x88x9220xc2x0 C. The glass temperature of the polyacrylate specifies the lower operating temperature of the material; below the glass temperature the polyacrylate is rigid and inelastic and the elastomeric properties of the material are lost. Suitable acrylates are alkylacrylates having 1 or preferably 2 or more carbon atoms in the alkyl chain. Methacrylates having a glass temperature low enough are alkylmethacrylates having 4 or more, preferably 8 or more, carbon atoms in the alkyl chain. These monomers can be used alone or in mixtures of two or more monomers. The glass temperature of the final product can be tailored by adding small amounts of monomers having fewer carbon atoms in the carbon chain to the above mentioned monomers. One can further use acrylates and methacrylates as co-monomers; which in addition to an ester bond have other polar groups such as alkcoxy or hydroxy groups. Examples of these are methoxy- and ethoxy-acrylate, hydroxyethyl- and hydroxypropyl-methacrylate. By using these co-monomers the oil resistance of the product can be improved. Also, small amounts of other non-acrylate monomers that are polymerizable by free radical polymerization techniques can be co-polymerized with the above mentioned acrylates and methacrylates.
In order to produce a material that is a thermoplastic elastomer the acrylate must be in the majority although the exact amount to be polymerized into the polyolefin depends on the exact polyolefin used and whether or not oil is added. Here, majority means at least 50%, preferably greater than 50%, more preferably at least 60%, yet more preferably at least 64%, and still more preferably at least 69%. According to this invention, a polypropylene based material needs 50-90% by weight acrylate when no oil or filler are added. Thus, the polypropylene represents 50-10% by weight. Without oil and filler addition the amount of acrylate can vary from 50-90% by weight for homopolyethylene, down to 20-90% by weight for polyethylene qualities which contain up to 30% by weight co-monomers. The Examples indicate the effect of the amount of acrylate on the softness of the final product.
Adding oil also softens the final product, thus reducing the amount of acrylate needed to obtain a particular softness. The amount of added oil can be 0-40% by weight in the final product and can be added with the acrylate and initiator, allowing penetration of the oil into the polyolefin-polyacrylate particles during the impregnation and/or polymerization. Alternatively, oil can be added to the reactor after the finalized polymerization and can be impregnated into the polyolefin-polyacrylate particles at an elevated temperature. Yet another way to introduce oil into the polyolefin-polyacrylate particles is in an extruder. Suitable oils are those normally used to soften rubber, e.g. paraffinic, naphthenic, aromatic and synthetic oils as well as plasticizers for thermoplastics such as dioctylphthalat.
Fillers can be added to modify the final product""s properties. For example, fillers can raise the operating temperature and rigidity. The filler can be added to the polyolefin-polyacrylate blend in the extruder or can be included with the polyolefin used as raw material for the polymerization. Conventional fillers such as talc, caolin, CaCO3 and silica can be used and can be 0-70% by weight in the end product.
The end product can also contain oil and fillers besides polyolefin and polyacrylate. Consequently, the amount of polyolefin and polyacrlate in the end product can vary within wide margins depending on the amount of oil and fillers used and also on the chosen polyolefin. If the polyolefin is polypropylene, the ratio of polypropylene/polyacrylate can be 0.1 to 2. If the polyolefin is polyethylene, the ratio can vary from from 0.1 to 5.
Some acrylates spontaneously form gels without any diacrylate use, for example butylacrylate, and may eliminate the need to use diacrylate for crosslinking. The need for diacrylate also depends on the degree of adhesion between the discrete dispersed polyacrylate phase and the continuous polyolefin phase. As the adhesion between the phases increases the tendency of the dispersed polyacrylate to agglomerate and build bigger phase structures decreases. For example, if polyethylene, which contains polar groups, is used the adhesion can be so good that only small amounts or no diacrylate at all is needed. On the other hand, if a homopolyethylene or polypropylene is used, the adhesion between the phases is low and the polyacrylate must be crosslinked with diacrylate in order to enable processing of the dispersed polyacrylate without agglomeration and forming large polyacrlyate blocks. Low adhesion can also lead to phase invasion where the polyacrylate becomes a continuous or at least a co-continuous phase with the polyolefin. Crosslinking is preferably done in the reactor with an acrylate having two or more double bonds that can interact with different polyacrylate chains. Examples of suitable crosslinking agents are hexanediol diacrylate or dimethylacrylate. Generally the crosslinking agent is 0-15% by weight, based on the amount of acrylate. Other monomers having two or more double bonds, such as divinylbenezene, can also be used.
Initiators conventionally used in free radical polymerization of vinyl monomers such as organic peroxides are suitable for the acrylate polymerization. Examples include benzoylperoxide,lauroylperoxide, t-butylperbenzoate, t-butyl-peroxy-2-ethylhexanate, t-butylperoxide, dicumylperoxide, di-t-butylperoxide, bis(t-butylperoxyisopropyl)benzene, t-butylperoxyisopropylcarbonate, 2,5-dimethyl-2,5-di-t-butylperoxyhexane, 2,5-dimethyl-2,5-di-(t-butylperoxy)hexyne-3, and azo compounds like azobisisobutyronitrile and azobisdimethylvaleronitrile.
More than one initiator can be used simultaneously so that the polymerization starts at a low temperature with a xe2x80x9clow temperature initiatorxe2x80x9d and continues with a xe2x80x9chigh temperature initiatorxe2x80x9d at a higher temperature. The amount of the intiator can be between 0.001 and 2% by weight, preferably between 0.1 and 1% by weight, based on 100 weight parts of monomer.
In principal, the production of this polyolefin-polyacrylate material can be made by the methods presented in the patent literature in which the acrylate and the initiator are first initiated into polyolefin particles and the acrylate is thereafter polymerized by elevating the temperature. The impregnation of the acrylate and the initiator can thus be made in the total absence of water, by adding some water, by adding water when more than half of the acrylate has been impregnated (these three methods are in principal described in the Finnish patents FI85496, FI86642 and FI88170) or in the presence of the total amount of water (as in US patent U.S. Pat. No. 4,412,938). Impregnation and polymerization can also be conducted simultaneously by slowly adding the acrylate and initiator to a water suspension containing polyolefin particles over the course of several hours and at an elevated temperature. (see German patent DE 2,907,662).
Finnish patent FI88170 presents an advantageous method whereby a maximum of about 65% by weight of acrylate is impregnated and polymerized into polyolefin in the polymerization stage. For softer elastomer, additional polyacrylate can be impregnated into the product obtained from the first polymerization stage, followed by a second polymerization. Using this approach the polyacrylate content can gradually be raised close to 100%. It is not necessary, however, to use totally independent or separate polymerizations. For example, near the end of the first polymerization, the temperature can be lowered to the impregnation temperature and the desired amount of acrylate and initiator can be pumped in. After these have been absorbed into the particles, the temperature is raised and the acrylate polymerized.
When polymerization is conducted in two or more stages and crosslinked polyacrylate is desired, the first polymerization is conducted without diacrylate. Here, the polyacrylate forms a discrete dispersed phase during the polymerization stage. During the other polymerization stage the added acrylate and diacrylate tend to migrate to the polyacrylate particles already formed in the polyolefin matrix, and crosslinking occurs there. This crosslinking is mainly between the added acrylate and diacrylate, but since this reaction occurs during polymerization in the presence of the existing polyacrylate particles, entanglements between the polymerizing and these preexisting particles are formed, creating physical crosslinks.
The final product is a thermoplastic elastomer with a continuous phase of polyolefin crosslinked to a discrete dispersed phase of a rubber-like polyacrylate. The polyacrylate phase is in the majority. This final product maintains its two discrete phases during melt-processing. Other properties include: a Shore A hardness value greater than 50, preferably greater than 60, even more preferably greater than 70, 80 or 90 (test method is ISO 48), a modulus 100% of at least 0 Mpa, preferably greater than 1 Mpa, more preferably greater than 2 Mpa, even more preferably greater than 3 Mpa (test method is 37/1 mm/min), tensile strength of at least 1.9 MPa, preferably at least 3.2 MPa, more preferably at least 5.1 MPa, even more preferably at least 6.3, 7.1 or 8.4 MPa (test method is ISO 37), an elongation at break of at least 75%, preferably at least 107%, more preferably at least 209%, even more preferably at least 354%, 449% or 528% (test method is ISO 37), and a tear strength of at least 0 kN/m, preferably 2 kN/m, more preferably at least 8 kN/m, even more preferably at least 8 kN/m, 16 kN/m or 20 kN/m (test method is ISO 37).
The polymerization product has especially good oil resistance, weather resistance and ageing resistance due to the polyacrylate elastomer. The properties of the thermoplastic elastomers produced according to this patent depend on the polyolefin used: homo, block or random polypropylene, homopolyethylene or polyethylene containing co-monomers. The choice of polyeolefin especially influences temperature resistance, chemical resistance and adhesion properties. The acrylate type, amount and crosslinking density affect the hardness, toughness and elasticity of the final product. Ethylene based products are characterized by good heat and oil resistance. Fillers, which can be added to the starting polyolefin, allow tailoring of the product""s properties.
The material produced of the current invention can be used in applications which other thermoplastic elastomers or conventional rubber is used, for example in the construction industry (sealing lists and packages), in the motor industry (protection bellow at power transmission points and interior material for instrument panels) and in the electrical industry (material for cables, contacts and different cases). This material can also be used for diverse mechanical articles like handles, wheels and sheaths.
The material can be processed by conventional processing methods used for thermoplastics, such as extrusion, injection molding and blow molding. Since polyolefin is the continuous phase, the material is well suited for co-extrusion with polyolefins. In processing, conventional additives like antioxidants, filler and oil can be added.